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Nuts'n'Bolts Utilities
There is a newer version: 6.2.0
/*
 * Copyright (c) 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020 David Berkman
 *
 * This file is part of the SmallMind Code Project.
 *
 * The SmallMind Code Project is free software, you can redistribute
 * it and/or modify it under either, at your discretion...
 *
 * 1) The terms of GNU Affero General Public License as published by the
 * Free Software Foundation, either version 3 of the License, or (at
 * your option) any later version.
 *
 * ...or...
 *
 * 2) The terms of the Apache License, Version 2.0.
 *
 * The SmallMind Code Project is distributed in the hope that it will
 * be useful, but WITHOUT ANY WARRANTY; without even the implied warranty
 * of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
 * General Public License or Apache License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * and the Apache License along with the SmallMind Code Project. If not, see
 *  or .
 *
 * Additional permission under the GNU Affero GPL version 3 section 7
 * ------------------------------------------------------------------
 * If you modify this Program, or any covered work, by linking or
 * combining it with other code, such other code is not for that reason
 * alone subject to any of the requirements of the GNU Affero GPL
 * version 3.
 */
package org.smallmind.nutsnbolts.validation;

import java.util.regex.Pattern;
import javax.validation.ConstraintValidator;
import javax.validation.ConstraintValidatorContext;

public class EmailValidator implements ConstraintValidator {

  private static final Pattern EMAIL_PATTERN = Pattern.compile("([a-zA-Z0-9_\\-\\.']+)@((\\[[0-9]{1,3}\\.[0-9]{1,3}\\.[0-9]{1,3}\\.)|(([a-zA-Z0-9\\-]+\\.)+))([a-zA-Z]{2,4}|[0-9]{1,3})(\\]?)");

  private Email constraintAnnotation;

  @Override
  public void initialize (Email constraintAnnotation) {

    this.constraintAnnotation = constraintAnnotation;
  }

  @Override
  public boolean isValid (String value, ConstraintValidatorContext context) {

    if (value == null) {
      return true;
    }

    if (constraintAnnotation.separator() == '\0') {

      return isAnEmail(value);
    } else {

      int lastIndex = 0;

      for (int index = 0; index < value.length(); index++) {
        if (value.charAt(index) == constraintAnnotation.separator()) {
          if (!isAnEmail(value.substring(lastIndex, index).trim())) {

            return false;
          }

          lastIndex = index + 1;
        }
      }

      return isAnEmail(value.substring(lastIndex).trim());
    }
  }

  private boolean isAnEmail (String possibility) {

    return EMAIL_PATTERN.matcher(possibility).matches();
  }
}